Process and composition for coating titanium surfaces



United States Patent PROCESS AND COMPOSITION FOR COATING TITANIUMSURFACES Leo Missel, North Hollywood, and James 0. Powell, Glendale,Calif., assiguors to Menasco Manufacturing pany, Burbank, Calif acorporation of California No Drawing. Application August 31, 1953 SerialNo. 377,706

10 Claims. (Cl. 204-38) This invention relates to the metal coating artand has particular reference to a process and composition for coatingthe surfaces of titanium and titanium alloys to permit subsequenttreatment, such as electroplating, of the same.

The primary object of this invention is to provide a novel process andcomposition for coating surfaces of titanium and titanium alloys with afilm of metal.

Titanium and titanium alloys are finding increasing uses andapplications, particularly in the aircraft industry, due primarily tothe excellent strength-weight ratios of the metal and its alloys.However, these materials exhibit tendencies toward galling, seizing andexcessive Wear when utilized in applications requiring motion betweencomponents such as in bearings, sliding assemblies and the like.Attempts have been made to produce a metallic bearing surface on membersformed of titanium and its alloys, but such attempts have heretoforebeen unsuccessful.

One of the principal objects of this invention is, therefore, to provide'a process for the treatment of titanium and its alloys to permit theapplication thereto of a hard coating having satisfactory bearingcharacteristics.

Another object of this invention is to provide a process for thetreatment of surfaces of titanium and titanium alloys to permit theelectroplating thereon of metallic films having characteristics ofadhesion, hardness, electrical conductivity, solderability, etc.,comparable to films produced on common metals by conventional methods.

Another object of this invention is to provide a process for chromiumplating the surfaces of titanium and titanium alloys.

Other objects and advantages of this invention, it is believed, will bereadily apparent from the following detailed description of preferredembodiments thereof.

Briefly, this invention includes the discovery that the surfaces ofarticles of titanium and titanium alloys may be provided with a metallicfilm by chemical deposition of the film brought about by immersion ofthe article in an aqueous solution of a mixture of hydrofluoric acid, asalt of the metal to be deposited, and a chromium compound, whichmetallic film permits subsequent electrolytic plating of the articles.It has been further discovered that good results can also be obtained byimmersion of the article in'an aqueous solution of a mixture ofhydrofluoric acid and a copper salt.

The following are specific examples of processes and compositionsembodying the present invention, but it is to be understood that it isnot intended to limit the invention to the specific steps set forththerein Example 1 Aircraft "landing gear parts forged of a titaniumalloy i ncluding5% chromium and 3% aluminum were cleaned by aconventional degreasing agent, rinsed and then imniersed in an acid etchbath consisting of 3 parts nitric acid and 1 part hydrofluoric acid. Thetemperature of "ice I Z the bath was F. and the time of immersion was 30seconds. The purpose of this step is to remove the gross oxide layerstypical of those found on forged titanium and titanium alloys. Whilebest results are obtained with the described acid mixture, other acidssuch as sulfuric, hydrochloric, hydrofluoric acid alone and mixtures ofsulfuric and hydrofluoric may be and have been used. The next step wasto water rinse the parts from the acid etch bath, and they were thendipped into a chemical deposition bath for a period of 60 seconds, thetemperature of the bath being maintained at 212 F. The bath comprised anaqueous solution of the following ingredients in the proportionsindicated:

Example 1a Grams/liter Hydrofluoric acid (anhydrous HF) 50 Zinc sulfate(ZnSO .7I-I O) 5 Sodium dichromate (Na Cr O 2H O) The parts were removedfrom the bath and water rinsed. The parts were then chromium plated in aconventional hard chrome electroplating bath at 3 amperes per squareinch for 4 hours, the bath temperature being maintained at F. Theplating solution consisted of 33.0 oz./ gal. of chromic acid and 0.33oz./ gal. of sulfate ion.

The thickness of the chrome plating obtained by the above process was0.0045 inch. The adherence of the plating under steel ball indentationtests and under specially designed bearing load tests was excellent andqualified the parts for use in heavy duty aircraft landing gear.

The chemical reactions and/or physical phenomena involved in thedeposition of the metallic coating prior to electroplating are notentirely understood. It would appear, however, that the hydrofluoricacid in the deposition bath set forth in Example la plays an importantpart in the removal of the oxide film during the deposition step. Thehydrofluoric acid may be formed in situ by the addition of a watersoluble salt of hydrofluoric acid and another acid, such as, forexample, sulfuric, hydrochloric, citric, phosphoric, etc.

The inclusion of the dichromate in the deposition composition of Examplela is also an important feature of the invention. It has been found thatthe dichromate makes possible a chromium plate which meets the rigidrequirements for heavily-loaded bearing surfaces. It is believed thatthis is due in part to a competitive reaction with the zinc salt whereintightly bound zinc remains adhered to the titanium, and the more looselybound zinc is redissolved, permittin tightly bound zinc to be depositedin the less adherent areas. Although somewhat less satisfactory, otherchromium compounds such as, for example: chromic acid; alkali metalsalts of a chromium acid such as sodium chromates, potassium andammonium chromates and dichromates and the hydrates of these compounds;and other water-soluble salts of a chromium acid may be substituted forthe sodium dichromate.

Heavy metal salts other than zinc sulfate have been successfully used inthe chemical deposition bath of Exilarly, other soluble salts of heavymetals such as, for

imately 2.5% to 5%.

example, copper chloride, copper acetate, silver sulfate, cadmiumchloride and nickel perchlorate may be used.

The production of maximum adherence of chromium plating appears to befavored by arelatively high content of sodium dichromate, varyingbetween approximately 10% to 30% by weight of the solution, a relativelylow zinc sulfate content, varying from approximately 0.5% to 2.0%, andhydrofluoric acid concentration from approx- However, even withrelatively Hydrofluoric acid. 0.01-15.0% (by-weight a as anhydrous HF).Chromic acid or soluble 'salt 0.1% by weight to satthercof; urationpoint. Heavy metal salt 0.01% by weight to'sata uration point. WaterBalance.

The temperature of the deposition bath of Example 111 may be varied fromabout 60- Feto the boiling point of the solution. The immersion time isnot critical, but for best results should be not substantially less than60 seconds, 'andcan be as long as 3 minutes or more.

Example 2 Titanium alloy parts were cleaned and acid etched'inthesamemanner asset forth in Example 1. They were then immersedforaperiod of 30 seconds in a deposition bath maintained at a temperature of212 F. consisted of an aqueous solution of the following ingredients inthe proportions indicated:

Example 2a Grams/liter Hydrofluoric acid (anhydrous) 10 Copper.sulfate225 After rinsing, the parts were plated in aconventional hard chromeplating solution at, 3 amperes per square inch for four hours, producinga plating of excellent appearance and having a thickness of 0.0055 inch.The adhesion of the chrome plate produced by the process of this example2 is not sufiiciently high to permit use of the .parts Where highbearing loads are experienced. However, the adhesion is excellent forrelatively low hearing loads, and for decorative plate and other lesscritical uses, it is more than adequate. p

Other copper salts such as, for example, copper chloride, copperacetate, copper bromide, etc., may be substituted for the coppersulfate.

Bestrresults are tions of copper sulfate and relatively low hydrofluoricacid concentrations. The immersion time is not critical but should notbe substantially more than 3 minutes or less than about 5 seconds. Theproportion of the ingredients of Example 2a may be varied as follows:

Hydrofluoric acid Here again, the temperature may be varied from about60 F. to the boiling point of the solution.

obtained by relatively high concentra 0.1 to 10% by weight as 4 aqueoussolution of from about 0.01% to about 15% by weight of hydrofluoricacid, from about 0.1% by weight to the saturation point of an alkalimetal salt of a chromium acid, and from about 0.01% by weight to thesaturation point of a soluble salt of a heavy metal selected from thegroup consisting of copper, zinc, silver, cadmium and nickel.

2. A composition for coating the surfaces of articles of titanium basealloys containing about 5% chromium and about 3% aluminum, consistingessentially of an aqueous solution of from about 0.01% to about 15% byweight of hydrofluoric acid, from about 0.1% by weight to the saturationpoint of a' compound selected from the group consisting of chromic acidand water soluble salts of a chromium acid, and from about 0.01% byweight to the saturation point of a soluble salt of a heavy metalselected from the group consisting of copper, zinc, silver, cadmium andnickel. g

3. Acomposition for coating the surfaces of articles of titanium basealloys containing about 5% chromium and about 3% aluminum, consistingessentially of an aqueous solution of from about 0.01% to about 15% ofhydrofluoric acid, from about 0.1% by weight to the saturation point ofsodium dichromate, and from about It will be readily understood by thoseskilled in the art that the process and composition of this inventionare not limited to the chromium plating operation. For example, partstreated as described may be given a copper plating from a conventionalacid copper bath, or maybe plated with any other conventional metal.

Thev above description and examples are intended to be illustrativeonly. Any modification of or variation therefrom which conforms to thespirit of the invention is intended to be included within the scope ofthe claims.

We claim: V a

1. A composition for coating the surfaces of articles of titanium basealloys containing about 5% chromium and about 3% aluminum, consistingessentially of an 0.01% by Weight to the saturation point of zincsulfate. 4. A process for coating the surfaces of articles of titaniumbase. alloys containing about 5% chromium and about 3% aluminum,including the step of immersing said articles in a bath consistingessentially of an aqueous solutionof from about 0.01% to about 15% byweight of hydrofluoric acid, from about 0.01% by weight to thesaturation point of a soluble salt of a heavy metal selected from thegroup consisting of. copper, zinc, silver;

cadmium and nickel, and from about 0.1% by weight to the saturationpoint of a compound selected fromthe group consisting of chromic acidand water'soluble salts of a chromium acid, to deposit the heavy metalonthe surfaces of said articles. v

5. A process for coating the surfaces of articles of titanium basealloys'containing about.5% chromium and about 3% said articles in a bathconsisting essentially of an aqueous solution of from about'0.01% toabout 15 by weight of hydrofluoric acid, from about 0.01% by weight tothe saturation point of a soluble salt of a heavy metal selected fromthe group consisting of copper, zinc, silver, cadmium and nickel, fromabout 0.1% by weight to the saturation point of a compound selected fromthe group consisting of chromic acid and water soluble salts of achromium acid, and maintaining the temperature of said bath betweenabout 60 F. and the boilingv point of' said solution, to deposittheheavy metal on the surfaces'of said articles.

6. A process. for coating the surfaces of articles of titanium basealloys containing about 5% chromium and about 3% aluminum, including thestep of immersing said articles in a bath consisting essentially'of anaqueous solution of from about 0.01% to about 15% by weight ofhydrofluoric acid, from about 0.1% saturation point of sodiumdichromate, from about 0.01% by weight to the saturation point of 'zincsulfate to deposit'zinc on the surfaces of said. articles. I

7. In a process for coating thesurfaces of articles of titanium basealloys containing about 5% chromium and about 3% aluminum,'the steps ofcleaning the articles to be coated, immersing the removing said articlesand, rinsing the same, immersing the articles in an aqueous solution offrom about 0.01% to about 15 by weight of hydrofluoric acid, from about0.1% by weight to the saturation point of sodium dichromate, and fromabout 0.01% by weight to the saturation point of zinc sulfate for a ondsat a temperature corresponding approximately to the boiling point ofsaid solution to deposit zinc on the surfaces of said articles, removingsaid articles and tin:-

alruninum, including thesteps of immersing by weight to the articles. inan acid etch bath,:

periodof about 60 secing the same, and electroplating said articles in aplating solution.

8. In a process for coating the surfaces of articles of titanium basealloys containing about 5% chromium and about 3% aluminum, the steps ofcleaning the articles to be coated, immersing the articles in an acidetch bath, removing said articles and rinsing the same, immersing thearticles in an aqueous solution of from about 0.01% to about 15% byweight of hydrofluoric acid, from about 0.1% by weight to the saturationpoint of a compound selected from the group consisting of chromic acidand water soluble salts of a chromium acid, and from about 0.01% byweight to the saturation point of a soluble salt of a heavy metalselected from the group consisting of copper, zinc, silver, cadmium andnickel for a period of about 60 seconds at a temperature correspondingapproximately to the boiling point of said solution, to deposit saidheavy metal upon the surfaces of said articles, removing said articlesand rinsing the same, and electroplating said articles in a platingsolution.

9. A composition for coating the surfaces of articles of titanium basealloys containing about 5% chromium and about 3% aluminum, consistingessentially of an aqueous solution of about 50 g./l. of hydrofluoricacid,

6 about 100 g./l. of sodium dichromate and about 5 gJl. of zinc sulfate.

10. A process for coating the suriaces of articles of titanium basealloys containing about 5% chromium and about 3% aluminum, including thestep of immersing said articles in a bath consisting essentially of anaqueous solution of about g./l. of hydrofluoric acid, about g./l. ofsodium dichromate, and about 5 g./l. of zinc sulfate to deposit zinc onthe surfaces of said articles.

References Cited in the file of this patent UNITED STATES PATENTS2,580,773 Heiman Jan. 1, 1952 FOREIGN PATENTS 378,872 Great Britain Aug.17, 1932 OTHER REFERENCES Handbook on Titanium Metal; 7th edition,Titanium Metals Corporation of America, New York, August 1, 1953, p. 92.

8. IN A PROCESS FOR COATING THE SURFACES OF ARTICLES OF TITANIUM BASEALLOYS CONTAINING ABOUT 5% CHROMIUM AND ABOUT 3% ALUMINUM, THE STEPS OFCLEANING THE ARTICLES TO BE COATED, IMMERSING THE ARTICLES IN AN ACIDETCH BATH, REMOVING SAID ARTICLES AND RINSING THE SAME, IMMERSING THEARTICLES IN AN AQUEOUS SOLUTION OF FROM ABOUT 0.01% TO ABOUT 15% BYWEIGHT OF HYDROFLUORIC ACID, FROM ABOUT 0.1% BY WEIGHT TO THE SATURATIONPOINT OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF CHROMIC ACIDAND WATER SOLUBLE SALTS OF A CHROMIUM ACID, AND FROM ABOUT 0.01% BYWEIGHT TO THE SATURATION POINT OF A SOLUBLE SALT OF A HEAVY METALSELECTED FROM THE GROUP CONSISTING OF COPPER, ZINC, SILVER, CADMIUM ANDNICKEL FOR A PERIOD OF ABOUT 60 SECONDS AT A TEMPERATURE CORRESPONDINGAPPROXIMATELY TO THE BOILING POINT OF SAID SOLUTION, TO DEPOSIT SAIDHEAVY METAL UPON THE SURFACES OF SAID ARTICLES, REMOVING SAID ARTICLESAND RINSING THE SAME, AND ELECTROPLATING SAID ARTICLES IN A PLATINGSOLUTION.